Conventionally, in electronic devices that are required to be thinner, such as calculators, portable telephones, and portable game devices, conductive rubber has been used as connectors between contacts of electronic components such as liquid crystal displays, microphones, and speakers and those of printed circuit boards. In this case, a holder for fixing the conductive rubber is prepared and the conductive rubber is inserted thereinto manually and is compressed to be in contact with the electronic components and the printed circuit board therebetween. Thus electrical contacts are obtained.
Thereafter, with an improvement in technology for mounting electronic components, solder mounting by SMT aimed to reduce assembly time has become the mainstream. Accordingly, for example, metal spring contacts compatible with SMT have been used often, but devices that are operated by hand, such as calculators, portable telephones, and portable game devices, have bodies that tend to be distorted or warped by force of people's hands. Inside a body, therefore, the contact part of the metal spring slides on an electrode of the electronic component that is in contact therewith, and thereby the treated surface of the electrode, for example, gold plating, is worn away, resulting in loss of electrical connection in the worst case, which has been a problem. With these situations, there have been demands for components that do not cause any damage to electrodes of electronic components inside the devices that are operated by hand, such as, calculators, portable telephones, and portable game devices, even when the bodies thereof are distorted or warped. Examples of components that can be used to achieve the object include a metal-integral conductive rubber component (Patent Document 1) and a conductive rubber (Patent Document 2) that can be solder mounted by SMT.
In the metal-integral conductive rubber component as described in Patent Document 1, a conductive rubber layer and an insulating rubber layer are laminated alternately and parallel to each other, the boundaries between the conductive rubber layer and the insulating rubber layer are integrated by cross-linking the conductive rubber layer and the insulating rubber layer, and a metal sheet is joined to at least one of the surfaces perpendicular to the electrical conduction direction of the laminate through a conductive adhesive layer made of a conductive rubber to integrate the metal sheet with the laminate. In this case, it is necessary to use a conductive adhesive and a metal foil that are relatively expensive. Moreover, the material of the conductive adhesive is different in material properties from the laminate of the conductive rubber layer and the insulating rubber layer. Particularly, there has been a problem in poor compression set, resulting in a problem of electrical connection reliability in the case of using it as a conductive rubber component.
The conductive rubber component as described in Patent Document 2 does not include such a conductive adhesive layer as that of Patent Document 1 but has a metal coating produced by sputtering to be integrated, which prevents the compression set from posing a problem. However, due to the material constitution of the laminate of the conductive rubber and the insulating rubber, it is not suitable for electrical connections that require lower resistance, which has been a problem. Furthermore, the material (any metal selected from gold, platinum, silver, copper, nickel, titanium, aluminum, and palladium or an alloy thereof) specified for the metal coating in the sputtering method to be employed tends to undergo chemical alteration due to the ionization tendency depending on the material. Accordingly, there is a problem of corrosion in a poor environment. There are some materials that do not tend to undergo chemical alteration, such as gold and platinum. However, when the metal coating is formed of such a material alone to have a thickness that allows it to be soldered, the material cost becomes expensive, which has been a problem. Moreover, since the object to be coated is a conductive rubber, it is difficult to increase the deposition rate. Therefore, when the time required for sputtering is taken into consideration, the cost for the coating process becomes expensive, which also has been a problem.